There are many instances in which it is desirous to have a sustained rate of release of a fluid such as ink, moistening fluids, adhesives and the like over an extended period and a large number of cycles. Examples of materials where such sustained rates of release are desirable include ink pads and ink rollers in inking systems. Examples of the use of ink pads are pads that are applied to print elements in order to transfer ink therefrom onto sheets made of paper, plastic, film and the like. Ink pads are used in devices such as postage meters whereby a postage indicia can be printed upon an envelope by contacting an ink laden pad with a print element having dies made of a metal, alloy, rubber, photopolymer, and the like and impacting the latter upon an envelope. Another form of an ink releasing mechanism is a roller. Examples where ink rollers are used are in labellers for price marking and postage meters. The ink roller serves as a source of ink for the labeller and postage meter, and the ink is transferred by appropriate mechanisms to a ticket, tag or label in the case of labellers and to a print element in the case of a postage meter as discussed previously.
Having a uniform, constant rate of release of ink has been a goal in the production of such devices for a long time. Obviously, the ideal situation is where the first ink impression and the last ink impression are of the same ink density. Clearly, this is an unlikely achievement in depleting devices. Nevertheless, it would be advantageous having the first and last inking impressions as close to one another as possible in terms of ink density. Ink density is clearly related to the amount of ink transferred. The amount of ink transferred is related to the rate of release from the roller or the pad. More specifically, this rate should be close to a constant or the change in the rate should be small. Further, because of a predetermined constant bulk volume dictated by the printing mechanism, prior ink rollers and pads provided a limited number of print cycles with a pay off of 15-25% of the total volume of ink impregnated in the foam.
Inks that are ideal for sustained release should have all the active ingredients (solvents, toners, dyes, etc.) moving as a homogenous phase. Currently, a constraint in ink making is the matter of toxicity since consideration must be given towards environmental factors. This is also true of inks that are used with dispensing pads and rollers. In the past, many inks have been used, as for example with rubber stamps, that have been found to have undesirably high toxic levels, but presently, wide spread efforts are underway to produce inks with solvents that are low in toxicity. For some applications the ink should also be non-volatile, and have low vapor pressure. In addition, the inks should also be non-destructive to the pads, rollers, print elements and other materials with which they come in contact.
There are certain inks that have coloring materials such as dyes and toners that are visually detectable and others with both coloring materials and fluorescent materials as well. In such systems, both the coloring materials and fluorescent materials should be dispensed slowly by the inking mechanism and at the same rate over a long term. An example of where inking and fluorescent materials are used in an inking pad or roller is the postage meter. A postage meter either will have an inking roller as is common in a drum-type postage meter and flat bed meters, or it can have a pad in a flat bed type postage meter.
A factor that works against sustained rates of release are the mechanisms in which they are used. In postage meters using rollers, for example, the mechanism involves constant engagement of the roller with the print element and such engagements cause non-uniform stress field across the rollers. The stress field applied on a roller is non-uniform and time dependent due to the foam/ink intrinsic properties. This stress is also dependent on the viscoelastic properties of the polymer, porosity and pore size of the foam and the amount of the ink available. As the ink depletes and the polymer is mechanically deformed simultaneously, the rate of ink release decreases.